Functional Control ESIE10-01
2–36 Part 2 – Functional Description
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Staging sequence This section defines which compressor is the next one to start or stop. In general, compressors with
fewer starts will normally start first, and compressors with more run hours will normally stop first.
Compressor staging sequence can also be determined by an operator defined sequence via set
points.
■ Next To Start
The next compressor to start must meet the following requirements:
Lowest sequence number of those compressors available to start:
■ If sequence numbers are equal, it must have the least starts.
■ If starts are equal, it must have least run hours.
■ If run hours are equal, it must be the lowest numbered compressor.
■ Next To Stop
The next compressor to shut down must meet the following requirements:
Lowest sequence number of the compressors that are running:
■ If sequence numbers are equal, it must have the most run hours.
■ If run hours are equal, it must be the lowest numbered compressor.
Compressor
capacity control in
cool mode
In Cool mode, evaporator LWT is controlled to within 0.2°C of the target under constant flow conditions
by controlling capacity of the individual compressors.
Compressors are loaded with a fixed step scheme. The rate of capacity adjustment should be
determined by the time between capacity changes. The farther away from the target, the faster
compressors will be loaded or unloaded.
The logic should project ahead to avoid overshoot, such that the overshoot does not cause the unit to
shut off due to evaporator LWT dropping below the target minus the Shutdown Delta T set point while
there is still a load on the loop at least equal to the minimum unit capacity.
Capacity of the compressors should be controlled so that when possible their capacities are balanced.
Circuits that are running in manual capacity control or running with active capacity limiting events
should not be considered in the capacity control logic.
The compressor capacities should be adjusted one at a time while maintaining a capacity imbalance
that does not exceed 12.5%.
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